A conventional overlock sewing machine, or the so called lock sewing machine, is provided with a main shaft rotated by a sewing machine motor, a sewing needle vertically moved by the rotation of the main shaft, and an overlooper and an underlooper swung by the rotation of the main shaft. Overlock stitches are formed on a cloth by swinging the overlooper and the underlooper in synchronization with the vertical movement of the sewing needle.
Aforementioned types of sewing machines are disclosed for example in JP-B-S58-19317 and JP-Y-H6-4875. In such overlock sewing machines, drive shafts for driving the overlooper and the underlooper respectively are arranged in a direction perpendicular to the main shaft which is disposed horizontally near the border of a bed and an arm. The drive shafts are respectively rotated in a reciprocating manner by 2 eccentric cams provided on the main shaft. The overlooper and the underlooper are swung via link mechanisms linked to the respective drive shafts.
Since conventional overlock sewing machines have a dedicated eccentric cam and drive shaft for the overlooper and the underlooper respectively, increased number of parts are required for the drive mechanism of the foregoing, thereby increasing the cost and the overall size. Furthermore, such construction requires laborious and troublesome installation and timing adjustment.
In such type of overlock sewing machines, thread engagement work for setting a looper thread to the underlooper need to be performed before starting the sewing operation which is relatively troublesome for the user. JP-Y-H7-19435 and JP-A-2624512 disclose an overlock sewing machine having a construction that renders a simplified looper thread engagement of the underlooper. Under such construction, the underlooper is mounted on a looper arm fixed to a looper shaft via a threading base. Also, a guide rail supporting a slider thereon is fixed to the threading base, allowing the slideable operation of the slider.
Upon engaging the thread to the underlooper, first, the user is required to pull open a bed cover of the bed. Then the looper arm is moved to the right most position so that the distal end of the underlooper is exposed to the exterior of the bed. Next, the user pulls out the slider and passes the looper thread through the threading eyelet on the distal end of the underlooper and engages the thread to the thread transferring member of the slider. Thereafter, the slider is pushed in the returning direction. The foregoing manual operation enables the looper thread to be engaged to a thread engagement portion in the base end of the underlooper.
However, in such construction, the provision of the threading base for thread engagement, the guide rail, and the slider increases the number of parts as well as the cost, consuming considerable time upon installation. Moreover, upon engaging the thread to the underlooper, after pulling out the slider and engaging the looper thread to the thread transferring member, the slider needs to be pushed back in. Thus, thread engagement work cannot be sufficiently simplified.